Some processors used in modern computer systems support a technology called hyper-threading. Hyper-threading enables a single physical processor to appear as multiple logical processors. This principle is illustrated in FIG. 1, in which one physical processor 6 includes two logical processors 7. Hyper-threading, therefore, allows a single physical processor to execute multiple code streams concurrently. The logical processors have some private state, but a portion of the state is shared. Current hyper-threading implementations support two logical processors, with the logical processors sharing the execution engine and the system bus interface. Each logical processor has some unique resources, such as general purpose registers, program counter and local Advanced Programmable Interrupt Controller (APIC) registers.
When hyper-threading is used in a processing system, some technique is required to schedule (allocate) the various processes in the processing system among the logical processors. A simple approach to using hyper-threading technology is to treat the logical processors as if they are physically separate processors. For example, this approach might involve scheduling processes across all logical processors as if they were physically independent processors. This works from a functional viewpoint, but since certain resources are shared, these processors can interfere with each other in terms of performance.